In technical applications, where shafts are rotated to drive a load, the shafts are normally held in bearings and very often the bearings wear, or some times the shaft wears, or both. Under any or all of such circumstances, there is a serious concern about malfunctioning of the related mechanical devices due to wear. For instance, in a turbo-generator set, there is a large shaft connecting the turbine with the generator. If the bearing or shaft wears, there could be a damaging effect on the generator or on the turbine, or both. The wear displacement of the shaft need not be large to cause damage. The shaft of the generator set with which the preferred embodiment of the present invention is employed is 28 inches in diameter and the user is interested in detecting a two mil variation, or displacement.
There have been various techniques employed to detect shaft displacement. One such technique has been to locate a velocity transducer in a seismic relationship with the shaft. If the shaft were wearing, its "wobble", or displacement, would move the velocity transducer up and down at some frequency rate, and with some amplitude. The velocity transducer comprises a mass which is mounted on a soft spring. The velocity transducer is bulky and is normally mounted some 15 to 20 inches from the shaft because of its large makeup. Because of the large distance from the transducer to the shaft it becomes necessary to employ a long probe which is undesirable. In addition, the velocity transducer must be mounted in the center of the shaft so that the loading factor against the soft spring is gravity.
For the foregoing reasons, the velocity transducer is considered less than a good solution for a radial displacement detector. In accordance with the present invention an accelerometer is employed which is not bulky and which can employ a short probe and which is spring-loaded so that it can be mounted at any angle around the periphery of the shaft. There has been one drawback with employing the accelerometer. It has been determined, in actual practice, that shafts have machine marks on their periphery and these marks each "bump" the accelerometer to provide a relatively high frequency vibration and, therefore, produce a relatively high frequency signal. For instance, if there were 20 machine marks located around the periphery and the shaft rotated at 60 HZ, then the accelerometer would be moved up and down at 1200 HZ and in such a situation, there are very often harmonics in the 10k to 14k HZ range. As will be discussed in more detail hereinafter, it has been found that an undamped accelerometer responds to said harmonics to provide a large amplitude factor or a large Q. Of course, such a condition leads to the generation of spurious signals. Electronic filtering circuits have been employed to help, and such circuits have helped to a certain extent but have not acted to completely remedy the problem.
With the present device, the resonant frequency of an accelerometer can be varied or adjusted so that it is below the frequency of oscillation or vibration caused by perturbations (such as machine marks) on a shaft, and yet can be adjusted so that it will provide a true response within a substantially usable linear range, to detect vibrations or displacements due to wear of the shaft or the bearings.